So, will it rain tomorrow? Add to myFT So, will it rain tomorrow? The Met Office has been trying to answer this question for 150 years. Have Britain’s weather scientists now cracked it? Read next June 24, 2011 by: Clive Cookson Noon on Thursday June 9 in the Met Office operations room in Exeter. From desk to ceiling, brightly coloured computer screens show past weather and future predictions. Showers speckle a rainfall radar map of the British Isles. A temperature chart shows tongues of warm orange air sticking into a pool of cool blue over the North Atlantic. 92 mm highest rainfall recorded in one hour, Maidenhead, July 12 1901 In the middle sits Martin Young, the chief forecaster, facing a dilemma about the weather three days ahead. He has known since the beginning of the week that an Atlantic depression was likely to reach Britain at the weekend, but now, with millions of people making plans, he must issue more specific guidance about when and where the rain is going to fall on Sunday. “There’s an unusually large difference between the [computer] models three days ahead,” Young explains.

The supercomputer at the European Centre for Medium-Range Weather Forecasts in Reading puts the incoming depression and its associated rain a full 300 miles further north than the Met Office’s own model. “We have techniques for merging models on screen,” he says. “I’ve blended the early part of one model with the later part of another one. But at the moment we’re leaning more towards the Met Office model.” That would mean rain reaching London at about 2pm on Sunday and northern England by nightfall – we’ll see later what actually happened. Clouds gather over weather instruments, used in conjunction with radar. For all the prodigious processing power and observational resources of the worldwide meteorological system, forecasting decisions still come down to human skill and experience. The Met Office would never rely solely on a computer-generated forecast, says Young, who has worked there for 32 years – moving from the old headquarters in Bracknell, Berkshire, to the new Exeter centre. For Young’s boss, chief meteorologist Ewen ­McCallum, today’s uncertainty about what will happen in three days’ time illustrates the improvement in forecasting over the past generation.

When he joined the Met Office 37 years ago, forecasters frequently faced similar or worse uncertainty about what would happen the next day. “A four-day forecast today is about as accurate as a one-day forecast was when I started,” says McCallum, in an accent as Scottish as his name. “Then, we had no operational access to weather satellites, no radar and very slow computers.” Public service Changing face of the Met Office As we walk round the operations room, talking to people who are preparing more specialised forecasts for particular groups, such as shipping and aviation, the sense of continuity and pride in the Met Office is evident. Many of those present have spent their whole working lives in its service. “I have been at the Met Office for 28 years; my father joined in 1950 and was a forecaster for 34 years,” says Rob Varley, operations director. “So the Met Office has been an intimate part of my life since before I was born.” Brian Golding, head of forecasting research and development, will soon celebrate 40 years at the Met Office. The raw materials in a successful forecast have improved beyond recognition over that period, he says, and will continue to do so in the future.

The first key ingredient is the fundamental physics of the atmosphere and how it interacts with oceans and land masses to produce weather. This is encapsulated in increasingly sophisticated models, as computing power grows. The £33m Met Office supercomputer – a twinned IBM Power 6 machine installed in 2009 and about to be upgraded – can carry out trillions of calculations a second. It sits in two huge halls, shrouded by what look like plastic shower curtains. These are intended not to preserve the modesty of the energy-guzzling machine but to reduce the need to cool in the immediate vicinity. Forecasts become more accurate as the model’s spatial resolution increases. The latest global model cuts the atmosphere into 70 horizontal levels, with a spacing of 25km between points at each level, though the resolution of this grid can be improved to as little as 1.5km for short-term forecasting over the British Isles. The other ingredient in forecasting is having ­accurate data to feed into the computer models. “The World Meteorological Organisation facilitates a remarkable global system for sharing data,” Golding says.

“Nowadays satellites provide the bedrock of our global observations, though radiosondes [weather balloons] are still used extensively and direct data from aircraft is becoming more important.” For UK forecasting, observations from the network of local weather stations remain essential, while 18 weather radars across the British Isles give forecasters a picture of where rain or snow is falling. One of the most important developments in recent years is ensemble forecasting. Because the observations are far from being a perfect description of the atmosphere – and because the atmosphere is in scientific terms a “chaotic” system in which very small errors in the initial state can lead to big errors in its development a few days ahead – the ensemble technique runs the computer model many times with slightly different starting conditions. Ensemble forecasting gives meteorologists a much better idea of the likelihood that a particular event will occur than a single computer run. “At the moment we can run the forecast 24 times for our ensemble,” says Golding. If the ensemble runs vary a lot, then the forecaster knows there is a lot of uncertainty about what will happen. If they cluster together, he or she will have more confidence in predicting a particular event.

One occasion on which this technique gave the Met Office enough confidence to issue a local severe weather warning three days ahead was the 2009 Cumbrian flooding. All the ensemble runs showed a stationary weather front over Cumbria embedded in a very moist south-westerly airstream – a recipe for prolonged downpours over the hills. In the event, Seathwaite recorded 31.6cm of rain on ­November 19, an all-time UK record. Bashing the weathermen is part of the British psyche Beyond seven days, the accuracy of forecasting takes a dive – at least in the changeable conditions of north-west Europe, one of the most difficult parts of the world for longer-range predictions. The Met Office currently offers the public forecasts up to a month ahead, couched in general terms, because day-by-day predictions become unreliable more than a week or so ahead. They are produced by the ensemble method, running the model many times and averaging out the results. Here is a typical example: “The generally unsettled conditions look to continue as June comes to a close, with further showers or rain likely to affect many parts of the country, but especially Scotland and Northern Ireland.

However, there should also be some drier, brighter and slightly warmer periods, these most likely in the south of the UK, but particularly across south-eastern parts of England…” 38.5 C highest recorded temperature in the UK, Faversham, August 10 2003 Until March last year, the Met Office stuck its neck further out by issuing seasonal forecasts. It stopped after public ridicule following the notorious “barbecue summer” forecast for the damp summer of 2009 and the failure to predict the cold winter of 2009/10. The trouble was that the nuanced predictions of the seasonal forecasts were spun first by Met Office staff and then again by the media. For instance, the forecast at the end of April 2009 said there was a 50 per cent chance that temperatures from June to August would be above average, a 30 per cent chance that they would be around average and a 20 per cent chance that they would be below average.

Chief meteorologist Ewen McCallum has spent 37 years at the Met Office Ewen McCallum, talking to journalists when the forecast was issued, could not resist commenting: “The chances of getting the barbecue out are much higher than last year.” And that quickly translated into “barbecue summer” headlines, although McCallum emphasised at the time that the seasonal prediction was based on probabilities and there was a substantial risk of its being wrong. However, the Met Office still sells seasonal forecasts to large commercial customers such as energy companies, which believe they can make worthwhile business decisions on the basis of probabilistic forecasts. Small businesses and individuals such as farmers and gardeners, who might also be sophisticated enough to make planting decisions based on the forecasts, lose out. Behind the scenes the Met Office has an intensive programme to make its seasonal and other long-range forecasts accurate enough for presentation to the public again in the future. Adam Scaife, who is in charge of the research, points to several improvements in the pipeline. -27.2 C lowest recorded UK temperature, various dates and locations in Scotland One is to fix a weakness of the current computer model: its poor representation of “blocking highs”.

These anticyclones stay almost stationary for weeks on end and so does the weather they generate. Blocking highs over northern Europe, which produce heat and drought in summer and severe cold in winter, have proved particularly troublesome for the British Isles. A “ground-breaking” new model predicts the formation and eventual disintegration of blocking highs much better, Scaife says, “because it has a better representation of the Gulf Stream. It is too expensive [in computer resources] to use routinely now, but we hope to put it into operation in a year or two.” A second improvement comes from working out the consequences for the northern hemisphere of the famous oscillation in the tropical Pacific Ocean between warm El Niño and cold La Niña. Although the impact in the southern hemisphere is dramatic, as Australia’s La Niña floods showed earlier this year, the indirect effects on Europe are only now emerging. The latest research shows that El Niño tends to produce high pressure over the polar regions of the north Atlantic and low pressure further south – leading to cold winters in northern Europe. The Hazard Centre for dealing with severe weather such as floods and blizzards Third, scientists at the Met Office and elsewhere are beginning to understand the effect of the 11-year solar cycle on climate.

When sunspots and other solar activity are at a minimum, the effect is similar to that of El Niño: more easterly winds and cold winter weather for Britain. “We now believe that [the solar cycle] accounts for 50 per cent of the variability from year to year,” says Scaife. With solar physicists predicting a long-term reduction in the intensity of the solar cycle – and possibly its complete disappearance for a few decades, as happened during the so-called Maunder Minimum from 1645 to 1715 – this could be an ominous signal for icy winters ahead, despite global warming. Moving beyond the next season, Met Office scientists are also working on annual and, particularly, decadal forecasts – the territory between weather and climate prediction. Business and government planners are used to working on timescales 10 years ahead, but horizons of 50 or more years, often discussed by climate change researchers, may be too long term. 33 Average number of tornadoes reported in Britain each year Accurate weather predictions, whether hours or years into the future, are just the first part of the forecaster’s job.

“Today it is much more about communicating the weather and its likely impact on people than doing a mechanistic forecast of tomorrow’s weather,” Golding says. To communicate effectively with the outside world – the public, commercial customers and government departments – good internal communications are essential. Besides the usual computer-based methods, a new means of personal communication has emerged since the move to Exeter: the Thursday “Street brief”. After lunch, Met Office staff emerge from their open-plan offices to congregate on the internal balconies of the building’s elongated atrium, an area known as the Street. At 1.45pm, forecaster Tom Morgan embarks on a detailed disquisition about previous weather and the Met Office’s success in predicting it – which has been good over the past week. World first Met Office leads the way Then, projecting charts onto the white wall across the Street, he moves to the future, expressing doubts not just about Sunday’s weather but about the very next day. “By midday tomorrow there’s a trough over southern Britain but we have a large degree of uncertainty, considering the short range,” Morgan says. He predicts heavy rain for Exeter early on Friday afternoon – correctly as it turns out.

The rest of the briefing includes notable weather events around the world, such as the first hurricane of the season in the east Pacific, and a review of Britain’s remarkable spring weather: the driest season ever recorded in parts of south-east England and East Anglia, and the wettest May on record in the Outer Hebrides. Though the Met Office is an agency of the Ministry of Defence, the atmosphere in its headquarters seems open and relaxed – with little evidence that the visitor is in a defence establishment. Privatisation, an option raised briefly last year, is off the agenda, at least for the time being, and the Met Office’s arm’s length “trading fund” status seems to be shielding it from the worst of the cuts threatening the MoD and other government departments. Nor do the staff seem too concerned about the pasting the Met Office has received recently in parts of the media. “Bashing the weatherman is part of the British psyche,” says McCallum. However, some of the recent antagonism is linked to the Met Office’s deserved reputation as a champion of research into climate change – and its scientists’ unrepentant calls for urgent action against man-made global warming.

Some climate-change sceptics seem to attack Met Office weather forecasts as a way of undermining its climate predictions. Still senior staff insist that, judging from their personal experiences of talking to non-meteorologists and from its surveys of public trust, the reputation of the Met Office is higher than ever. “Our trust scores are about 82 per cent, which is phenomenal for any organisation,” says Varley. “I find it heart-warming that, when it comes to the crunch, people trust the Met Office.” So what happened on Sunday June 12, three days after the FT’s visit to Exeter? Anyone who was attempting outdoor activities in England and Wales will remember that the rain arrived sooner and moved north more quickly than forecast. In London it started to pour before breakfast, preventing any play in the men’s tennis final at Queen’s Club, which had been brought forward by two and a half hours in a fruitless effort to beat the weather. Clearly it was not the Met Office’s best moment of 2011, but whether you regard it as a noteworthy forecasting failure depends on your perspective.

“We picked up the general signal that it was likely to be a miserable wet day five or six days previously – and that was right,” McCallum tells me afterwards. While such a forecast would have been a triumph a generation ago, people expect much more today. Whether more accurate forecasting will outpace expectations in the years ahead remains to be seen. The outlook is bright, but no one in the Met Office expects to win universal love and approval. Clive Cookson is the FT’s science editor. To comment on this article please e-mail magazineletters@ft.com Sample the FT’s top stories for a week You select the topic, we deliver the news. Select topic Met Office moves out of Bracknell and into Exeter - Wales Online News Met Office moves out of Bracknell and into Exeter THE Met office yesterday began issuing all of its 850 daily forecasts from a new state-of-the-art complex aimed at improving forecasting. Share Get daily updates directly to your inbox + Subscribe Thank you for subscribing! Could not subscribe, try again laterInvalid Email THE Met office yesterday began issuing all of its 850 daily forecasts from a new state-of-the-art complex aimed at improving forecasting.

The operation moved from 11 sites in Bracknell, Berkshire, its home since 1960, to a base in Exeter, Devon. Chief executive Peter Ewins said yesterday that the move "is absolutely the centre of our strategy. "The premises in Bracknell were getting old and decrepit. We needed to raise the game in terms of information technology and modern techniques. "We are by far the most accurate weather forecasting operation in Europe, probably in the rest of the world. "To do that you have got to invest in people, supercomputing and support structure." The Met's supercomputer makes two thousand million calculations a second. In four months the centre's next-generation supercomputer will be about three times as powerful. Like us on Facebook Most Read Most Recent Most Read Most Recent S-Z S-Z (abbr) Sand, visibility 5000m or less. (as used in aviation reports, forecasts etc.) SALR (abbr) Saturated adiabatic lapse rate. The rate of cooling (variable) of a saturated air sample rising in the atmosphere.

(see "Stable and unstable air masses" ) Saturation The condition air reaches when it contains the most water in the vapour state that it is capable of holding at any particular temperature. If any more vapour is injected into the sample (or if the sample is cooled), then condensation will occur. SC (abbr) Stratocumulus (SC in METAR/SIGWX charts etc., Sc otherwise); a low-level cloud type, varying from thin, well broken layers with little impact for aviation/general weather, to deep, sometimes unstable character giving rise to persistent PPN, and a risk of moderate turbulence & moderate (some situations severe) icing. SCT (abbr) Scattered (3 or 4 oktas); cloud amounts used in aviation reports, forecasts etc. (of historical note, SCT used to mean 1 to 4 oktas, until the introduction of 'FEW' on revamp of the METAR code in the 1990's). Seasons In mid-latitudes, we are used to the idea of the four seasons: spring, summer, autumn and winter.

For climatological 'accounting' purposes, these are defined using three calendar month blocks thus:- March, April & May = spring; June, July & August = summer; September, October & November = autumn and December, January & February = winter. (For more, see "How are the seasons defined?" ) Seclusion process During the process of rapid cyclogenesis (q.v.), the standard ' Norwegian ' theory of development leading to an occluded front is not appropriate. What appears to happen is that the original cold front becomes weak/ill-defined (close to the low centre), and a new cold front appears further to the west. (This is effectively what has been drawn in the past as a 'back-bent' occlusion). So, what happens to the warm air associated with the warm frontal zone near the low centre? Around and immediately to the equatorward side of the low, it becomes trapped or 'secluded' from the rest of the development in a discrete region enclosed by relatively colder air encircling the development - a so-called 'seclusion'.

(This is therefore a different process from that producing the classical 'occlusion' whereby warm-sector air is lifted by the advancing cold air.) (see "What is the Shapiro-Keyser clyclone model?" ) Seeder/feeder mechanism When very moist (e.g. tropical maritime) air flow is forced to rise over upland areas, thick layers of stratus or stratocumulus cloud form. As noted elsewhere, these 'orographic' clouds of themselves produce relatively little rainfall (in a thermally stable environment). If however rain is already occurring from medium layer cloud (thick altostratus, nimbostratus) [seeder clouds], it will have to fall through the low-level [feeder] cloud, with collision/collection processes markedly enhancing the net rainfall rate at the surface. This effect often produces prolonged heavy rainfall in the warm conveyor regime within a warm sector, particularly if the system is slow-moving. Sensible heat transfer The transfer of heat by conduction and convection (i.e. it can be 'sensed' or detected directly). SEV (abbr) Severe (as in SEV ICE, for severe icing).

Severe Gale The definition of a 'Severe Gale/Force 9' is strict for operational (UK) forecasting for maritime purposes. Either the mean (10 minute) wind must be 41 knots or more, up to 47 knots; or the gusts must be 52 knots or more, up to 60 knots. The term will also be heard on broadcast weather forecasts, although it's arguable that the general population cannot be expected to know what this definition is, and the practice now is to explicitly forecast gust values rather than just relying on the adjective 'severe' to imply possible problems. (See also Gale , Storm and notes at the Beaufort wind scale .) SFUK Part of the WMO 'header' code used in bulletins that carry atmospheric reports, more commonly known as 'sferics, or 'SFLOCS'. (See "What are sferics?'" ) SG (abbr) Snow grains; as used in aviation weather reports. SH (abbr) Showers; as used in aviation weather reports/forecasts.

Shapiro-Keyser cyclone model See Relative vorticity Shortwave Short (!) for short-wavelength upper trough): A minor trough of small amplitude moving at speeds varying from 'steadily' to 'rapidly' through the long-wave upper pattern. Often best detected and monitored in water vapour imagery, and associated with development or de-stabilisation of the synoptic pattern. (See here ) SIGMET Issued by meteorological offices responsible for aviation forecasting. When significant flight/weather events are observed or forecast, then a SIGMET is issued by the office responsible (Bracknell [ to end-August 2003]/ Exeter [from start September 2003] for London, Scottish and the Shanwick Oceanic FIR's (Flight Information Regions); Dublin for the Shannon FIR) for such as embedded (EMBD) or frequent (FRQ) cumulonimbus (CB) or thunderstorms (TS); severe icing (SEV ICE) in frontal cloud; severe clear air turbulence (CAT) etc. Issued when there is a high degree of confidence, and for a short (usually max. 4hr) period only.

Significant Wave height The average of the highest one-third waves observed at a point and is approximately equal to the wave height an experienced observer would visually estimate for a given sea state. SIGWX (abbr) Significant weather, as in significant flight weather charts for route planning. Singularities Climatologists have always been alive to the fact that similar weather patterns/types occur at certain times of the year with varying degrees of regularity - an annual ' singularity '. For a while, before dynamical methods of long-range forecasting were used, singularities were very popular, though controversial. The best known (in the British Isles) are Buchan's spells and Lamb's singularities. SKC (abbr) Sky clear (as used in aviation weather reports, though from 2005, it should NOT be used in METAR reports). Sleet Strictly (by WMO regulation), this is defined as rain and snow falling in a mixed fashion, or snow that is melting as it lands. Unfortunately, in North America, it has a different definition where it is used to denote ice pellets - a totally different phenomenon.

SN (abbr) Snow (as used in aviation weather reports, forecasts etc.) SNOCLO (abbr) Runway/airfield closed due to snow cover. Snow Index (Eden Winter Snow Index): Philip Eden has defined as follows: ' Add together the snow depth in centimetres for all days with snow-lying at 0900Z. Treat a slight cover as 1cm, but ignore less than 50% snow-cover. Thus one morning with a 10cm cover, and ten mornings with a 1cm cover, would each score 10. The units, for the sake of argument, might be called "cm days" '. This can be used as a 'running-total' through a particular winter to compare different areas of the country (alongside such as numbers of days of snow-lying at 0900, numbers of days air minimum temperature < 0.0C (or other threshold) etc., or to put winter 'snowiness' in historical context. [ to see the procedures for reporting falls of snow - see here ] Snow-line (in forecasting) Often taken to be the 528 dekametre thickness contour line in the U.K.

It is useful, but rather a crude guide as to whether snow will fall at sea level. Meteorologists will prefer to use other parameters, such as the 850-1000 mbar partial thickness, or the wet bulb freezing level, but even these must be used with care. [ See the Thickness FAQ ] SNOWTAM (more correctly - runway state report) A group added to the end of a METAR (q.v.) which gives information relating to ice / snow (and other 'slippery') conditions ON THE RUNWAY. (See this section of the site) Solar radiation Short-wave electromagnetic energy from the sun. Source region Extensive areas where air remains in the same place long enough to acquire the characteristics of an air mass . Spanish Plume A catchy name applied to what is in reality quite a complex process producing the conditions necessary for severe local storms over maritime N.W. Europe. Strictly, the 'spanish plume' is the warm/dry ex-Saharan air, that has passed over the Iberian peninsula; been lifted by forced ascent (due to near-jet level forcing), cooling and moistening and producing outbreaks of thundery rain from medium level cloud.

Initially providing a 'lid' (see ' loaded gun ') which inhibits deep/vigorous convection, its breakdown allows the sudden release of potential instability, with the fuel for the subsequent severe storms being provided by air of a high theta-W value often running in from the SSE. Thunderstorms, often severe, are most likely within the tongue of highest theta-W air (> 18 degC or so), and where there are low-level forcing agents: e.g. isobaric troughing, sea breezes, coastal convergence etc. SPECI See METAR Specific heat The amount of heat energy required to raise the temperature of a specified mass of a substance by unit measurement of temperature. The specific heat of liquid water (at 0°C) is 4218 J deg-1 kg-1. For ice, it is approximately half this value. Specific humidity The mass of water vapour in a unit mass of moist air. Split-frontal type A situation where the vertically thick/cold-top upper cloud (usually producing the significant rain) has moved well forward, and moves notably faster than the classically analysed surface front (wind shift, dew point drop etc.).

With dry air over-running the rearward feature, the precipitation on the surface cold front is often light & 'drizzly'. Comparison of IR imagery (showing the sharp rearward boundary of the upper front), with the VIS imagery (showing the break from stratiform to broken, or cumuliform cloud structures), will identify such features very well. Sometimes an upper cold front will be analysed on UK Met Office charts when such a 'split' is well-marked. (See the FAQ here ) SQ (abbr) Squall(s) (as defined below - used in aviation weather reports). Squall (SQ) A squall is differentiated from a gust by its greater duration: generally lasting for several minutes before decaying again. Squalls are often associated with the passage of fronts, particularly cold fronts, or well defined troughs, or with the 'gust front' from a well defined/mature supercell Cb. To qualify as a line squall, other marked changes are often observed, e.g. change of wind direction, fall of temperature etc. The following definition is used when estimating wind speeds using the Beaufort scale of wind speed: "....

a sudden increase of wind speed by at least three levels of the Beaufort scale, the speed rising to F6 or more and lasting for at least one minute." SS (abbr) Sandstorm (vis generally < 1km); as used in aviation weather reports, forecasts etc. SST Sea surface temperatures. The subject of much discussion regarding reliability, methodology etc. For basic synoptic forecasting, accuracy to within 1 degC is fine; for climate change studies, tenths of a degree are of vital importance: such differences are easily introduced using differing methods of measurement e.g. bucket versus engine intake. SSTA Abbreviation for Sea Surface Temperature Anomalies, i.e. the difference between a short-period analysis (e.g. 5-day, 7-day or monthly 'snapshot' of a particular area of ocean) and the long-period 'normal' for a specified period such as 30 years. SSTA's are (or should be) a key component in any reliable long-range forecasting scheme.

ST Stratus (ST in METAR, aviation charts etc., St otherwise); a low-level cloud type, varying from thin, well broken fragmentary pockets of cloud to deeper, overcast and extensive layers giving rise to hill fog and occasional drizzle. Stable air Air is stable when an air parcel sinks or rises to its original position, when the force that initially moved it is no longer operating. [ See "Stable and unstable air masses" ] Sting jet In a small fraction of mid-latitude cyclonic storms, highly damaging surface winds are observed that cannot be attributed to the 'normal' gusts produced within an intense gradient flow, nor to those due to suspected tornadic development. It is thought that a zone of strong winds, originating from within the mid-tropospheric cloud head of an explosively deepening depression, are enhanced further as the "jet" descends, drying out and evaporating a clear path through snow & ice particles, the evaporative-cooling leading to the air within the jet becoming denser - helping to accelerate the downward flow.

The "jet" hits the surface as a relatively narrow zone of highly damaging winds (~ 80kt / ~150km/h, or higher) running around the southern flank (Northern Hemisphere) of the parent low: these high winds (at the surface) are found just ahead of the hook-like tip of the cloud stream which is being wrapped around the southern semi-circle of the low - the shape of which in satellite cloud imagery gives rise to the name "Sting Jet" (allusion to a scorpion's tail). [refs: Browning, Clark, Hewson; thanks to the latter for help with this entry.] STNR Stationary, as used in aviation forecasts, SIGMET's etc. Storm The definition in any good dictionary usually involves a mention of a 'strong wind' but also couples the term to such as thunderstorms, hail, heavy rain etc. When used within the UK Met Office Shipping Forecast, High Seas forecasts (and associated Gale and Storm Warnings), then 'Storm/Force 10' is strictly defined as either the (10 minute) mean wind 48 to 55 knots, or gusts 61 to 68 knots. (See also comments at Severe Gale ).

Storm surge When persistent, severe gales (usually stronger), markedly low atmospheric pressure* and geographic 'funnelling' of the wind-driven sea water are combined with astronomically high tides, then the resulting storm (or tidal) surge can cause coastal sea levels to rise several metres above the astronomically predicted level, with inundation of low-lying areas. Notable examples in regions bordering the North Sea occurred in 1099, 1236, 1287, 1421, 1697 and 1953. The North Sea is particularly prone to such events because it is shallow relative to the open Atlantic - often the source region for storm-driven waters - and its depth decreases still further towards its southern/narrow end. (* a decrease in pressure of roughly 10 mbar produces a sea-level rise of about 10cm.) Stratocu-Cugen Stratocumulus (Sc) formed (generated) by the spreading out of Cumulus (Cu), hence Sc cugen.

There are three main mechanisms by which this cloud form develops: (a): on a morning of cumulus formation, when there is a well-defined 'lid' (inversion) to convective development, and that inversion is moist, then cumulus development is arrested and 'spreading-out' occurs ..

see ' Overconvection '; (b): at the end of a day of cumulus (or cumulonimbus, Cb) development, as surface-based thermals weaken, the convective towers lose vigour, with subsidence beginning to dominate and the Cu (Cb) 'flattens-out' towards or around dusk; (c): around vigorous Cu/Cb towers, there is always subsidence - leading to small-scale, subsidence-formed stable layers adjacent to the convective towers - some of the cloud taking part in the shower/thunder development 'leaks out' sideways underneath these small-scale inversions, again leading to Sc cugen (or Sc cbgen in the case of Cb) Stratosphere (Latin,stratum=layer) A layer of high stability (in thermodynamic terms) such that air motion is primarily horizontal, although near the boundary with the troposphere (q.v.), marked vertical motion can occur (forced by jet-stream actions), which are important in driving developments in the troposphere.

(See the main FAQ here ) Streamlines Rather than drawing isobars (lines joining places of equivalent mslp), or contours (lines joining places of equivalent geopotential height), it is sometimes better to describe graphically the wind flow by drawing lines with arrows showing the direction of the wind at any level. Often used on / near the surface. Although long used in tropical / sub-tropical areas, ( where isobaric analysis is of dubious value ), streamlines are also very useful at mid-latitudes, for instance to determine likely areas of convective activity due to convergent triggering. Sublimation The process of a solid being transformed directly to a gas or vice-versa. (See Latent Heat ) Sunshine recording The 'classic' method of recording sunshine duration has relied on the use of a glass globe to focus the rays of the sun onto a medium (usually stout card) that is scorched when the sunshine is strong enough. By adding up the length of the scorch marks (which are related to bright sunshine duration), the total sunshine for the day can be assessed.

This method has been used for over a century: the instrument based on this principle which is in common use is the Campbell-Stokes recorder (CSR); one of the drawbacks of this unit is that is over-estimates, due to "burn-spread", the sunshine duration on days of strong but intermittent sunshine. There are other problems, not least that a human is needed to change the card daily and assess the burn pattern. (See the Observer's Handbook for more on this). In recent years, sensors have been developed which measure direct solar radiation above a defined threshold (currently set at 120 W/m^2), and these units can be left unattended and require no day-to-day intervention, with the results automatically fed into standard weather reports both hourly and over an aggregate of 24hr. (See here ): the instrument chosen by the UK Met Office is the Kipp and Zonen sensor (KZ). Comparisons confirm the long-held belief that the CSR over-estimates 'true' sunshine duration primarily when solar elevation is large and cloud well broken.

The difference between the methods (CSR > KZ) may be as much as 15 - 20% in the summer, falling to less than 10% in mid-winter. These differences must be acknowledged when comparing sunshine data, particularly when looking for 'records' or trends. Supercell storm See the main FAQ here . Supercooled water Liquid water at temperature below 0°C. Sustained wind The standard (WMO) averaging period for the surface wind for synoptic reports is 10 minutes. However, for use in tropical storm forecasting, and more generally in North America and the Caribbean region, a one-minute period is used: these winds are referred to as sustained winds in tropical bulletins. A rough conversion is given by: SUSTAINED (1 MIN MEAN)=1.14 * WMO-STANDARD (10 MIN MEAN). Sutcliffe development theory In 1947, R.C.

Sutcliffe published his seminal paper relating surface developments (in terms of vorticity forcing) to the patterns to be found on thickness charts, and developing an equation that could quantify such in terms of the thermal wind in the layer (500-1000hPa) and the vorticity diagnosed objectively across the layer and at the surface. The thermal wind appears in each of the equation's three terms, confirming the subjective observation that the stronger the thermal (tighter thickness gradient), the more intense is the subsequent synoptic development. [ For more on all this, see HERE . ] Swell Wind waves that have travelled out of the area in which they were generated or can no longer be sustained by the wind in the generating area. (see also Wind waves ) SYNOP (from 'synoptic'... see the main FAQ here ) A fully coded version of a meteorological report from a weather reporting station - in groups of 5 figures. For a good site which deals with decoding SYNOP data, visit Dave Wheeler's web site at: - T (strictly Frontal T-Bone) See " What is the Shapiro-Keyser cyclone model?

" TAF Terminal Aerodrome Forecast (also known formerly as a TAFOR) A meteorological forecast for a specific airport/airfield for a period covering 9 to 24 hours in 'self-briefing' code. TC (abbr) Tropical cyclone; used in aviation reports, forecasts etc. TCU Abbreviation used in the METAR/TAF code (as defined by ICAO), for a cloud type that meteorologists would call cumulus congestus (Cu con) - cumulus clouds which are characterised by marked 'sprouting' limbs, and which often extend to a high altitude - certainly well above the zero degree level. The bulging upper part is sometimes said to resemble a cauliflower. The cloud type is important for aviation as they can generate significant icing and turbulence in flight, and heavy precipitation, reduced visibility on/near the surface. (see also Cb ) TEMPO Temporarily; a temporary fluctuation in (aviation/airfield) conditions expected to last less than one hour in each instance and not occur in total during more than half of the period indicated. Tephigram Takes its name from Temperature and Entropy (formerly noted by the greek letter 'phi').

Used in the UK Met Office to plot upper air soundings, and assess such as instability, depth of moisture/cloud layers, fog points, temperature of free convection, condensation level etc. Designed by Napier Shaw early in the twentieth century. Other thermodynamic diagrams are used by other services (e.g. the pseudoadiabatic or Stüve chart [ common on web sites ]; skewT/log(p) diagram: ( see also the entry for Thermodynamic diagrams. ) Terrestrial radiation Long-wave (infra-red) radiation emitted by the earth (and other components of the earth-atmosphere system). Thermal steering Mid-latitude, mobile surface features (frontal depressions, mobile highs etc.) are often found to 'follow' the pattern of the thickness field over them. In particular, classic warm-sector depressions can be forecast to move in the direction of the thermal wind over the wave tip, with a speed 4/5 of the average thermal wind within 300 nautical miles (5 deg. of latitude) of the centre. As development takes place, the thermal field becomes increasingly distorted and the rule is less applicable.

[Strictly speaking, it is the surface centres of positive and negative vorticity forcing that are being steered, rather than the feature itself. Sutcliffe quantified this as one of the terms of his development equation. ] Thermal wind A theoretical (vector difference) wind that relates the magnitude of the horizontal temperature gradient in a defined layer to the real winds that blow at the top and base of that layer. The speed of the thermal wind is proportional to the temperature gradient. (see the thickness FAQ ) Thermodynamic diagram (also known as an Aerological diagram)A graphical plot of the observations of temperature and humidity, against pressure, as obtained via a Radio-Sonde ascent, or derived by sensing returns of radiant energy using an artificial satellite. Many diagrams are in current use, the three most often found are described here . Theta-W From the greek letter 'theta' and subscript 'w', used to denote wet bulb potential temperature (q.v.) - one of a group of pseudo-conservative (q.v.) properties of air masses. Thickness The difference in height between two layers in the upper air.

The most commonly used being the thickness between 500 mbar and 1000 mbar, and normally expressed in dekametres. The larger the value of thickness, the warmer the column of air (warm air expands). (See also " Thickness: what is it? ") Thunderstorm The World Meteorological Organisation defines as follows: " One or more sudden electrical discharges, manifested by a flash of light (i.e. lightning) and a sharp or rumbling sound (i.e. thunder)". Cumulonimbus (Cb) clouds are required, providing the necessary 'factory' for the vigorous updraughts / downdraughts, which in turn generate the enhanced charge-separation mechanisms within the cloud, and thus the marked positive and negative areas found within a Cb. Positive charge is found concentrated in the upper parts of the cloud, and negative in the lower, but just above the zero degC level, with small, but important positive charge areas in the base area, just below the zero degC level. (for the meteorological conditions necessary for a thunderstorm to occur [ in mid-latitudes ], see " Conditions for thunderstorms ".

Thundery rain A phrase used in weather forecasts when some quite sharp bursts of rain are expected from large areas of unstable medium cloud - with some 'rumbles' of thunder; the thunder not the most significant feature of the situation, and no well developed thunderstorms with associated hail, squally winds etc., are expected. Thundery showers A phrase akin to "wintry showers" which has crept into the language because many forecasts have severe word constraints. If well-developed and 'vigorous' thunderstorms are expected, then the forecast should say so, but when you only have 4 words to play with, some short-hand phraseology is necessary on a showery day with perhaps some scattered 'rumbles' of thunder. Time Standard The international meteorological community use UTC (or GMT) as the standard 'clock' for observations and forecasts. The 'day' is based on 00UTC and 12UTC, with multiples of 6 hours used for main observations. However, some countries (e.g. Australia), will use slightly differing conventions. When making an observation, it is important to state what 'clock' you are using, e.g. local time, GMT, etc.

Tornado(TN) A vortex extending upward from the surface (land or sea) at least as far as the cloud base (with that cloud base associated with deep moist convection), and is intense enough at the surface to do damage at one or more points along its path. (Doswell, C.A. III, 2001 / via: ~doswell/ ... this will change in the near future) TPC Tropical Prediction Center (part of NCEP ) also the National Hurricane Center (NHC) for the United States. For a wealth of information relating to tropical storm meteorology, visit the NHC site at:- TREND A two-hour 'significant weather' forecast appended to a routine (or special) weather report intended for aviation purposes. (see also METAR ). Triple-point The point where a cold, warm and occluded front all meet. Given the right upper-air structure, a discrete area of low-pressure can form here. Tropopause The (usually) abrupt change from falling temperatures with height in the troposphere, to near-uniform, or rising temperatures in the stratosphere. For coding purposes, defined as the lowest level at which the lapse rate decreases to 2 degC/km or less.

(with caveats to rule out lower level inversions.) (See the FAQ entry on the various levels of the atmosphere, here ) Troposphere (Greek, tropos=turn ) lowest layer of the atmosphere, with an average depth of 16 to 18 km around the equator, 9 to 12 km temperate latitudes and well below 9 km much of the time in arctic regions. There is a general fall of temperature with height (i.e. a positive lapse rate), with an average value of some 6.5 degC/km (or 2 degC/1000ft). (See the FAQ entry on the various levels of the atmosphere, here ) Trough A feature on a weather map where mean sea level pressure (or upper contour heights) are lower than surrounding areas of the atmosphere, with a 'V' shape to the isobars/contours evident in the pattern. Often associated with unsettled/cloudy weather, but not always. (see also upper trough , and " What is a trough? ". Trough disruption The process whereby part of an upper trough moves 'forward' (usually west to east), leaving a portion of the old trough behind.

(See anticyclonic trough disruption and cyclonic trough disruption .) Trough extension If an upper trough or thermal trough develops a markedly increased amplitude it is said to have undergone meridional extension. The change is usually defined in terms of a latitude change of a defined contour or thickness isopleth. TS (abbr) Thunderstorm (as used in aviation weather reports, forecasts etc.) TURB (abbr) Turbulence (as used in aviation weather reports, forecasts etc: e.g. MOD TURB). Turbulence Atmospheric motion that shows irregular and random motion over very small distances and over short intervals of time. U UKMO (or UKMetO) United Kingdom Meteorological Office. The HQ, (and central computing / NWP facility) was formerly located at Bracknell, Berkshire, UK., but since late summer of 2003 is now in Exeter, Devon [ together with the Hadley Centre for Climate Prediction and Research]. (NB: they now like to be called 'the Met Office'). Visit the web site at:- Undercut See Cold undercut Unstable air An air parcel will continue to move away (rise or fall) from its original location when the initial force exerted on it is removed.

[ see the FAQ entry on stable and unstable air masses . Upper Air Normally taken to be the levels between 850 mbar (about 5000 ft), and 200 mbar (39000 ft). The most common heights used being 500 mbar (18000 ft) and 300 mbar (30000 ft). Upper fronts It has long been recognised that the simple 'Norwegian model' (q.v.) of a frontal zone extending with a defined slope from the surface to the top of the troposphere did not often fit the observed weather experienced on the ground or by aircraft in flight. Often, bands of precipitation which were not ideally tied to the classical front were observed, and with the coming of satellite imagery, structures could be detected which are now classified as upper fronts. On actual/prognostic charts issued by meteorological centres (strictly these are surface charts), such features are shown using the classical symbology, but the triangles (cold fronts) and 'bobbles' (warm fronts) are not filled in, and are termed upper cold and upper warm fronts respectively.

Upper Ridge A ridge on an upper air chart - evidence of warm air in depth through the troposphere. Upper Trough A trough on an upper air chart - evidence of cold air in depth through the troposphere. V (abbr) Volcanic ash: as used in aviation reports, forecasts etc. VC (abbr) In vicinity: used in aviation reports, forecasts etc., e.g. VCFG, fog in vicinity but not at airfield. Veering See Backing Vertical vorticity Synoptic-scale vorticity about a vertical axis, caused by horizontal differences in wind speed (the 'shear' term), and / or horizontal 'curving' of the air-flow (the 'curvature' term). It is often assessed at the 500 hPa level (around 5.5 km). This property is linked to divergence / convergence at upper levels, which in turn will lead to enhancement of vertical air movement - key to synoptic developments. (See also Vorticity ; Horizontal vorticity .) Violent Storm This term is only used in bulletins for shipping, and associated Gale/Storm Warnings.

A 'Violent Storm/Force 11' is defined as the (10 minute) mean wind of between 56 and 63 knots. (Gusts not defined) (See also comments at Severe Gale ). VIS Visible imagery ... see the FAQ entry on satellite imagery . Visibility [ horizontal ] (from the Observer's Handbook ) " the greatest distance at which a black object of suitable dimensions can be seen and recognized against the horizon sky, or, in the case of night observations, could be seen and recognized if the general illumination were raised to the normal daylight level. With the advent of automatic sensors to detect this parameter, the WMO re-defined the above in terms of the 'meteorological optical range' (MOR), which is given by the path-length over which a light source of known intensity/colour is reduced to a certain percentage of the original flux. Visibility [ reports ] As used in SYNOP , SHIP and METAR observations: the value reported as the visibility in surface reports has for many years been the lowest visibility (as defined above) that an observer can determine.

So, for example, if the general value were 6 to 9 km, but in one particular direction (say towards a major town or over adjacent coast) it was as low as 3000 m, then it would be this latter value that was recorded, e.g. 3000 m. Until 2003-2004, this was also the scheme adopted for METAR reports world-wide. However, the concept of 'prevailing' visibility has been introduced for these latter, and can be taken to be the dominant visibility (applicable to at least 50% of the local horizon, continuously or otherwise), with separate groups for significantly lower values reported as required. For more on this, see the page relating to the METAR code. Visibility [ vertical ] Used in aviation reports to assess how far upwards a surface-based observer can 'see' through fog or snow. (See VV ). Volcanic effects The effect on the atmosphere due to a major volcanic eruption depend critically upon the plume maximum-altitude and its sulphur content.

Unless the plume penetrates into the stratosphere (and does so for a substantial length of time), material only affects the troposphere , and will be washed-out by precipitation (rain, snow etc.): in such cases, the effects will be regional and short-lived. In broad terms, sulphate aerosols penetrating to the stratosphere in quantity warm the stratosphere (due to absorption) and cool the troposphere (by radiation back-scatter), but the effect is highly size-dependent; only the smallest particles lead to long-term near-surface cooling. VOLMET Voice weather broadcast for in-flight briefing, often using automated interpretation of weather bulletins. Vorticity A measure of the 'spin' of a portion of a fluid - in our case, of atmospheric particles. Vorticity in a cyclonic sense is designated 'positive', and in an anticyclonic sense is designated 'negative'. In synoptic meteorology, we often only consider vorticity in a horizontal plane - i.e. the 'spin' behaviour of air particles as they move along in the atmospheric flow as depicted on classical 'weather maps': vertical vorticity.

However, the vertical component of vorticity is important in the study of tornadoes for example: this is horizontal vorticity. (See also: Absolute vorticity ; Horizontal vorticity ; Relative vorticity ; Potential vorticity ; Vertical vorticity .) VRB (abbr) Variable wind direction, as used in aviation weather reports, forecasts etc. VSP (abbr) Vertical speed, as used in aviation weather reports, forecasts etc. Particularly used with respect to forecasts of mountain wave motion. VV (abbr) Vertical visibility, as used in aviation weather reports, forecasts etc., e.g. VV002, vertical visibility 200ft. Where vertical visibility cannot be determined (or realistically forecast), then the group will be seen as VV///. W WAFC (abbr) World Area Forecast Centre - there are two such centres covering high-level / intercontinental air routes across the globe: "London" and "Washington" (actually located at Exeter, UK & Kansas, USA respectively).

In addition, RAFC (Regional Area Forecast Centres) are provided by various major weather forecasting services around the world to cater for medium-level work on a continental/sub-continental scale, though increasingly even this work is being concentrated on just a few centres. Purely low-level work (roughly below 10000ft) is the prerogative of national weather services (NWS's), or equivalent military formations. All charts issued by WAFC's, RAFC's & NWS offices are produced to standards agreed between member states of the International Civil Aviation Organisation (ICAO). For more on SIGWX charts see here . Warm advection The replacement of cold air by warm (usually) due to horizontal movement of air-masses. The process can be gradual or abrupt: the latter often being marked by a 'warm front' on a synoptic chart. Warm anticyclone This feature has a warm core (relative to surrounding air), with cold / dense air dominant in the upper part of the troposphere / lower stratosphere, contributing to the net high pressure observed at the surface. Has a deep circulation (i.e. 'high' / ridge found on charts extending to tropopause levels), with that tropopause high/cold etc.

Often associated with major long-wave blocking patterns. However, note that 'warm' anticyclones can have cold air trapped in the lowest layers under the surface inversion, and can of course be the means of advecting cold air (if it was there in the first place) around it's periphery. (See Anticyclones ; Cold anticyclone ) Warm occlusion If air (at the surface) behind an occluded front is warmer than the air it is displacing (a typical example being when a winter-time Polar Continental 'block' is being eroded by an incursion of Polar Maritime air), then the front is known as a warm occlusion. The occlusion is shown on synoptic charts as a linear extension of the warm front; the cold front leaving the occlusion/warm front at a fairly sharp angle. (see also Cold occlusion) . Warm-front wave A secondary disturbance, often accompanied by a shallow closed-low circulation, that forms at some point on a marked warm frontal boundary a good way (at least 1000 km) from the parent (occluding) depression. Once formed, it moves quickly away from the parent depression (in the Northern Hemisphere east or southeastwards).

Although not common, they are often responsible for considerable forecast errors, and are of particular importance in winter (snow-situation) forecasting as mild, maritime air attempts to displace a cold, continental blocking anticyclone. (See also cold-front wave ). Warm sector The area between a warm and cold front in the classical Norwegian model frontal system. Warnings - UK Public Service The Met Office is responsible for providing advice and warning of forthcoming Severe Weather within the United Kingdom. There are various categories; see here . WBPT See Wet Bulb Potential Temperature WDSPR (abbr) Widespread, as used in aviation weather reports, forecasts etc. Wet Bulb Freezing Level Because evaporative cooling is so important in rain versus snow forecasting (see the main FAQ entries, here and here ), the wet-bulb 0degC level is a better guide to snow-risk level than the actual ZDL . The wet-bulb is the lowest temperature to which air can be cooled by evaporating water into a sample, which is a mechanism often important in determining whether it will rain or snow at or near the surface.

However, note carefully that advection, both cold and warm must be taken into account - the former is particularly important in marginal situations in the vicinity of a well-marked cold front. (see also " Downward penetration of snow " in this Glossary). Wet Bulb Potential Temperature (often abbreviated to WBPT, or 'theta-W') A relatively conservative property within any one air mass that is derived from the temperature and humidity values of a particular air sample for a particular level: usually 850 or 500 hPa. Very warm/very humid southerlies for example (in NW Europe) would have typical 850hPa WBPT values well in excess of 16 degC, and perhaps as high as 20 degC or more; polar maritime air streams would have values typically 5 to 10 degC, but these values would be much lower in the depths of winter. Wet day A period of 24hr, conventionally beginning at 09UTC, during which precipitation of 1.0mm or more has been recorded. (See also Rain day ). WF Abbreviation for warm front.

Wind shear The change in wind direction, or speed, or both, either in a vertical or horizontal plane. Vertical shears are important in the study of convection, particularly for severe storm development; horizontal shear, particularly speed shear, contributes to relative vorticity terms in synoptic development. Wind waves (sea) Waves generated by the wind blowing over the surface of the ocean. (See also Swell ). Wintry precipitation (often used as 'wintry showers') When the air temperature is close to zero deg.C (either side), it is sometimes easier to use this shorthand term for showers producing soft hail, sleet, snow, 'sleety-rain' etc. Frowned upon by proper meteorologists but a useful term nonetheless. (However, we try to avoid it when the showers are much heavier, and are expected to give significant snowfall.) WKN (abbr) Weakening, as used in aviation forecasts, particularly SIGMETs. WR (abbr) Used in the uk.sci.weather newsgroup to report a 'weather event'; entered in the subject line thus: [WR] snow becoming heavy at Tilehurst 1540UTC.

The use of this abbreviation allows those who do not wish to read such reports to ignore them during 'busy' weather periods. (See also OBS ) WS (abbr) Wind shear, as appended to METAR reports etc. This abbreviation should only be used to indicate actual (or forecast) conditions experienced by aircraft in the atmospheric boundary layer. WV Water vapour imagery...see the FAQ entry on satellite imagery . X Y Z Z Zulu time (GMT/UTC), as used in aviation reports, forecasts and more generally by the meteorological fraternity used to military usage. It is gradually being ousted though by UTC (or sometimes just UT). ZDL Zero-degree (celsius) Level. A somewhat better description of this variable than ' freezing' level . (q.v.) Zonal A predominantly west-to-east airflow is termed zonal (and an east-to-west airflow is negative zonal).The strength of the flow in any sector may be expressed in terms of a zonal index given by the difference in average contour height between two latitude circles through the sector.

Question: Which public service moved from Bracknell in Berkshire, to Exeter in Devon in 2003?
Answer: The met office